Bitumen is produced from oil sand such as is found in the Fort McMurray region of Alberta, Canada. The oil sand is mined and has been commercially recovered using hot water processes which generally require slurrying the oil sand with at least heated water or steam. The slurry is conditioned and the resulting froth separated from the solids portion for recovery of the bitumen therefrom.
The bitumen containing froth is typically diluted with a hydrocarbon diluent, such as a naphthenic or paraffinic solvent, to reduce the viscosity and to aid in separating the bitumen from water and solids contained in the froth. Separation of the bitumen from the solids and water typically results in a bitumen-rich stream and a solvent-containing tailings feedstream.
In order to improve the economics of the bitumen recovery processes and the environmental impact of tailings disposal, solvent recovery apparatus and processes, generally known as tailings solvent recovery units (TSRU) are employed to recover the solvent from the solvent-containing tailings stream prior to disposal. The recovered solvent is typically recycled for use in the froth treatment processes. Conventional solvent recovery is typically effected by increasing the surface area of the solvent-containing tailings feedstream by creating a thin film of feed using internals such as shed decks or by retaining a pool of solvent containing tailings within the vessel for sufficient time, typically at least 5 to 15 minutes, to permit solvent release from the pool.
U.S. Pat. No. 6,712,215 to Scheybeler describes a solvent recovery vessel having pairs of nozzles located within the vessel. Each pair of nozzles is arranged horizontally in a 180-degree, opposed relationship. The nozzles have a first inlet for receiving solvent-containing tailings and a second inlet for receiving steam. A single outlet discharges a mixed steam/tailings stream. Adiabatic expansion of the steam, when contacted with the tailings, carries the discharge stream at high speed toward a centre of the vessel. The opposing orientation of the nozzles causes the discharge streams to collide at high speed forming small droplets in the vessel. The large surface area of the droplets facilitates release of the solvent from the liquid phase to the vapour phase. Shed decks located below the collision zone are heated using a steam sparger located beneath the shed decks and act to spread the droplets into thin layers thereon to facilitate heat transfer resulting in further solvent release. Another steam sparger is located in a lower portion of the column and further aids in stripping residual solvent from the liquid within the vessel. Pressure in the vessel is maintained at sub-atmospheric conditions through use of a steam eductor.
Canadian Patent 2,272,035 to Mocal Energy Limited et al (Syncrude) utilizes a vacuum flash TSRU vessel at sub-atmospheric pressure, preferably 30-35 kPa, to vapourize hydrocarbon diluents, including both naphthenic and paraffinic solvents, from the tailings stream. Steam is introduced into a pool of liquids which accumulate in the bottom of the vessel to vapourize residual diluent and water. The vapours are removed from the vessel and are condensed to recover the diluents for recycling.
Canadian Patent 2,272,045 to Mocal Energy Limited et al (Syncrude) teaches introduction of froth treatment tailings into a near atmospheric steam stripping TSRU vessel having a plurality of interior, vertically-spaced shed decks. The liquid tailings stream is evenly distributed over shed decks creating a thin film of the liquid feed on an upper surface of the shed decks. Steam is introduced below the shed decks and flows countercurrent to the tailings stream in order to vapourize the hydrocarbon diluents from the thin film of feed material. The vessel is preferably maintained at near atmospheric conditions (about 95 kPa).
Canadian Patent 2,353,109 to Shell Canada Limited et al teaches recovery of a paraffinic solvent, typically C5/C6, through introduction of solvent-containing froth treatment tailings to a TSRU vessel that is free of internals. The feedstream is heated by indirect heating or direct steam injection to about 75-90° C. prior to introduction to the vessel through an inlet device. The inlet device induces a pressure drop to atomize and disperse the liquid components of the stream and thereby minimize foaming. An impingement surface and co-injection of an inert gas to minimize foaming is also contemplated. The TSRU vessel is operated at slightly above atmospheric pressure, preferably 150 kPa-200 kPa. A pool of liquid and solids is maintained in a lower portion of the vessel to provide sufficient residence time to permit solvent to vapourize therefrom. The pool is agitated to ensure the solids, particularly the solvent-laden, less dense asphaltenes, and the liquids, are well mixed therein and to provide shear for reducing the particle size therein. The solvent is recovered as an overhead vapour stream. A bottoms stream may be recirculated into the same vessel or pumped to a second, identical vessel. The second vessel is typically operated at a lower absolute pressure (50-70 kPa) and at a temperature of about 50-70° C.
Canadian Patent Application 2,454,942 to Fort Hills Energy Corporation teaches introduction of solvent containing forth treatment tailings to a TSRU vessel that is free of internals and comprises a “pump-around” circuit to avoid formation of an asphaltene “mat” on top of the liquid and solids pool, to suppress foam formation and to increase solvent recovery. A pool of liquid and solids is maintained in the lower part of the vessel and a high downward liquid flowrate through the vessel is maintained by recycling a portion of the tailings, using the pump-around circuit, from the bottom of the vessel and back into the vessel. Shear from the recycle pump is useful to break up asphaltene flocs and liberate solvent from the tailings. The solvent is recovered as an overhead vapour stream. The TSRU vessel is operated at sub- atmospheric pressure. A two-stage process is also contemplated.
Canadian Patents 2,587,166, 2,613,873 and 2,614,669 to Imperial Oil Resources Limited teach steam stripping TSRU vessels having internals (shed decks) with feed streams that contain asphaltenes. Both one stage and two stage processes are claimed. Steam is introduced below the internals or directly into a pool of liquids in the vessel and, in all cases, no agitation is utilized to mix the contents of the pool. At least a first TSRU vessel is operated at an absolute pressure between 100 kPa-200 kPa and at a temperature of between 75° C. and 100° C. A second TSRU is contemplated and is operated at a pressure lower than the first TSRU, being about 20-200 kPa and preferably 25-100 kPa and at a temperature of between 75° C. and 100° C. Additional TSRU vessels are contemplated to be added in series.
There remains significant interest in the industry for apparatus and processes which result in even more efficient and effective solvent recovery, particularly for feed streams which contain paraffinic solvents and asphaltenes resulting therefrom and to meet ever more stringent regulations regarding minimizing solvent loss and tailings disposal generally.